Reduction of metal carbonyls via electron transfer. Formation and chain decomposition of formylmetal intermediates

Article abstract of DOI:10.1021/om00136a015

Electron transfer to metal carbonyls such as Cr(CO)₆ and Fe(CO)₅ affords the corresponding 19-electron anion radicals which undergo facile hydrogen atom transfer with trialkyltin hydrides. The yields of the resultant formylmetal carbonyls are limited by a radical-chain decomposition, the mechanism of which is established by a detailed study of the formyldirhenate complex (OC)₅ReRe(CO)₄CHO^-Bu₄N⁺ (IV). The chain process in Scheme I leading to the hydridodirhenate V is induced either thermally by azo initiator or photochemically with light of λ > 400 nm. High kinetic chain lengths and quantum yields of >300 are measured. Reductive methods involving initiation at an electrode surface or with sodium anthracene also promote the chain decomposition, but the turnover numbers are limited. Detailed kinetics analysis of the cyclic voltammetric data reveals the presence of two mechanistic pathways - one involving a radical-chain process (Scheme I) and the other an electron-transfer process (Scheme II). The radical-chain process for the decomposition of formylmetals is retarded by hydrogen atom donors. The latter is shown by the unusual stabilization of a variety of formylmetal species that are otherwise quite transient. Synthesis and decomposition of formylmetals by homolytic mechanisms involving facile hydrogen atom transfers are thus microscopically related processes.